Polypropylene resins are relatively cheap and have excellent thermal and mechanical properties, and so have been used in a wide variety of the fields. In general, however, a propylene homopolymer has high rigidity, but is inferior in impact resistance, especially low-temperature impact resistance. As a composition containing the propylene homopolymer improved in low-temperature impact resistance, there has been employed in each of industrial fields including automobiles and household appliances, a propylene block copolymer composition wherein the propylene homopolymer is initially produced and subsequently an ethylene-propylene random copolymer component is produced.
Those propylene block copolymer compositions which have been used hitherto are excellent in impact resistance, but inferior in transparency to homopolymer and have a high whitening on impacting. As a method to improve the disadvantage of whitening on impacting in the propylene block copolymer, there have been proposed the method of increasing the ethylene content in the copolymer and the method of adding polyethylene to the propylene block copolymer composition. Both methods are excellent in improving the whitening on impacting, while lowering the transparency of the product.
JP-A-5-331327 discloses a polymer composition comprising a propylene block copolymer composition with the specified ratio of the intrinsic viscosity of a propylene homopolymer component to that of an ethylene propylene random copolymer component.
JP-A-6-145268 discloses a polymer composition with the specified intrinsic viscosity of a propylene homopolymer component, the specified ratio of the intrinsic viscosity of a propylene homopolymer component to that of an ethylene propylene copolymer component and the specified ethylene content in the ethylene propylene random copolymer component.
JP-A-56-72042 discloses a polyolefin resin composition comprising an ethylene-propylene block copolymer and an ethylene polymer.
JP-A-57-63350 discloses a propylene copolymer composition comprising a propylene-ethylene block copolymer and an ethylene-alpha olefin copolymer.
Those polymer compositions have been more improved than conventional propylene polymer blends in respect of the impact-whitening resistance and transparency from the measured result of haze for the injection-molded articles of the polymer compositions, but more improvements have been desired for the polymer compositions. In addition, blending various components may bring about unbalance of the properties due to scattered dispersibility of each component in the final product.